CI: Run tests on GitHub Actions

[amotl]

Hi there,

this patch will make the whole test suite run on GitHub Actions. In this manner, contributions will be easier to handle than before as it will provide a reasonable safety net to assure maintainers that nothing significantly breaks.

It might also encourage users contributing new features to write corresponding test cases. The outcome from this will also be emphasized by adding a respective badge to the main README.rst file.

With kind regards,
Andreas.

/cc @einsiedlerkrebs, @poesel, @thiasB, @wetterfrosch, @ClemensGruber, @MKO1640, @WSt89, @nznobody, @rohlan, @msweef

[amotl]

With f3a78ae, the test matrix will be expanded to Python 3.7.

Also, with 91fcc85, the test matrix will be expanded to run against Mosquitto 2.0 as well. This will prevent havocs like regressions coming from things like #97.